THE REDOX STATE OF Fe AND S AT KAMA’EHUAKANALOA SEAMOUNT AND PRIMITIVE MANTLE ƒO2

Whether the mantle is heterogeneous or homogeneous with respect to oxygen fugacity (ƒO₂) is important because it can place constraints on the transportation of oxidized or reduced materials through the interior of Earth. The ƒO₂ of the mantle that feeds ocean island basaltic (OIB) volcanism is heterogeneous and produces basalts with fO₂ values ranging from greater than arc basalts to values similar to mid-ocean ridge basalts. The cause for this variation remains a topic of debate, with two hypotheses related to petrogenic processes (differentiation and degree of melting) as well as two related to mantle source characteristics (inherited from recycled slabs and or primitive lower mantle). To test between these hypotheses, we present Fe³⁺/∑Fe, and S⁶⁺/∑S ratios (XANES) of pristine glassy pillow basalts erupted at Kama’ehuakanaloa Seamount (formerly Lō’ihi), the youngest of the Hawaiian Island chain volcanoes. Kama’ehuakanaloa glasses have ³He/⁴He ratios 20-30 times greater than air, as well as negative µ¹⁸²W anomalies, which are both compositional parameters diagnostic of a primitive mantle source.

Kama’ehuakanaloa glasses studied here have 5.18-8.25 wt% MgO, are recovered between ~950-2400 mbsl, and are relatively un-degassed (apart from 5 samples) with respect to H₂O (0.019-0.113 wt%) and S (850-2570 ppm). These glasses have 0.158-0.206 Fe³⁺/∑Fe and 0.145-0.384 S⁶⁺/∑S, with calculated ƒO₂ from Fe³⁺/∑Fe of +0.12 to +0.60 ∆QFM. These Fe³⁺/∑Fe span much of the range observed in other OIB, including at Mauna Kea and Kilauea. However, the S⁶⁺/∑S of Kama’ehuakanaloa glasses are higher than previously observed at Mauna Kea. Fe³⁺/∑Fe and S are positively correlated, a typical record of sulfur degassing. While previous studies have concluded that Kama’ehuakanaloa glasses are relatively un-degassed with respect to S, this must be carefully re-evaluated. If they are indeed un-degassed, it may suggest a component in the Hawaiian plume that has high S and ƒO₂. These glasses also have Sr-Nd-Pb, He, and Os isotopic compositions which define involvement of DMM, EM1, and FOZO/PHEM, and one or more of these components could explain the high Fe³⁺/∑Fe and S values.